The major objective of this project is to determine those factors which initiate growth in the mammary gland. In particular, the first phase of this study was to isolate and characterize a small protein from serum which is capable of initiating growth in the mammary gland of the mouse. We have succeeded in the partial purification and characterization of mammary stimulating factor (MSF), a mitogenic peptide, isolated from serum which initiates growth in mouse mammary epithelium and mammary tumors. Using ion exchange chromatography, gel filtration, and isoelectric focusing, MSF was purified 250-fold from porcine serum. It is a heat stable protein of molecular weight 10,000 to 10,4000 with an isoelectric point of 5.5 to 6. MSF initiates DNA synthesis in vitro in mammary epithelium to a greater extent than in mouse mammary tumor cells (CZF), 3T3 cells, or chick embryo cells. Comparison of the biological, physical, and immunological properties of MSF with other established growth promoting peptides suggests that MSF is a unique serum factor. We have found that the material not only initiates DNA synthesis, but also permits cell cycle traverse resulting in net cell growth.